Various dialkyl benzenes, such as ethyltoluene and diethyl benzene, are used as important precursor compounds from which the corresponding vinyl aromatic monomers are made. The resulting monomers, i.e., vinyltoluene and divinylbenzene, are essential to the production of a variety of styrenic polymer materials. Additionally, xylene is a useful dialkyl benzene used in the production of terephthalic acid which is an important intermediate in the synthesis of polyester fibers and films.
In the case of diethylbenzene and ethyltoluene, the para isomer is the most useful intermediate, with the ortho isomer being highly undesirable. Because of the undesirability of the ortho isomer, expensive distillation techniques must be employed prior to dehydrogenation of the ethyltoluene and diethylbenzene.
At present, many alkylbenzene conversion processes include processing steps wherein the aromatic substrates which are to be converted are contacted under conversion conditions in the presence of catalysts. Both single and multiple bed catalyst processes are well known in the art. An important property to be considered in the selection of the catalyst is the selectivity of the catalyst to the desired product. A subcategory of the selectively of the catalyst to the desired product is the selectivity of the catalyst to the desired isomer of the desired product, for example, "para-selectivity".
Various aluminosilicate type zeolite catalysts, including those known as "ZSM" catalysts, have been reported to be suitable for selectively producing para substituted benzene derivatives upon being modified for that purpose. One problem with these types of catalysts, however, is that they are subject to rapid deactivation in the presence of even small amounts of water. Rapid deactivation means that a high rate of conversion of reactants to products cannot be maintained over a long period of time thus requiring expensive catalyst changeouts or regeneration procedures which greatly reduce the efficiency of the overall process. Thus, in using such catalysts it is sometimes necessary to reduce the moisture content of the feed stock materials prior to their introduction to a conversion zone.
Another problem with the aforementioned aluminosilicate type catalysts is that they must often be modified with "promoters" to obtain significantly increased para-selectivity. In other words, these types of catalysts have little or no intrinsic para-selectivity, i.e. the catalysts must be modified before they will produce a product in which the para isomer is present in an amount greater than in a thermodynamic equilibrium. The thermodynamic equilibrium concentration for isomers of ethyltoluene is about 31.5% para, about 50.2% meta and about 18.3% ortho at normal operating conditions for vapor phase alkylation.
More recently, catalysts characterized as crystalline silica polymorphs prepared in accordance with specified procedures and known generally as "silicalite" type catalysts, have been discovered to be useful in aromatic conversion processes. It has already been disclosed that these catalysts have intrinsic para-selective properties. It would therefore be most useful to have a catalyst showing even more para-selectivity, even in the presence of steam.
As noted earlier, the para isomer of the dialkylbenzenes is the most useful intermediate, with the ortho isomer being the most undesirable. A need exists, therefore, for a process for selectively producing para dialkylbenzenes in amounts greater than that present in a thermodynamic equilibrium.